Liposomes are closed, microscopic structures having concentrically arranged amphiphilic double layers which separate aqueous compartments from one another. Because of their similarity to cell membranes, liposomes have been investigated for many years as a multifunctional carriers and transporting systems for biologically-active substances, including prokaryotic and eukaryotic genes (D. Arndt, I. Fichtner (ed.): Liposomen Darstellung--Eigenschaften--Anwendung (The Preparation of Liposomes, Their Properties and Use), publ. Akademie-Verlag, Berlin (1986); G. Gregoriadis (ed.): Liposomes as drug carriers: Recent trends and progress, publ. John Wiley and Sons. Chichester (1988); G. Lopez-Berestein, I. J. Fidler (ed.): Liposomes in the therapy of infections diseases and cancer, publ. Alan R. Liss, N.Y., (1989); C. Nicolau, A. Gudd: Liposomes as carrier of DNA, Critical Rev. Therapeutic Drug Carrier Systems 6, 239-271 (1989)).
The work dealing with liposomal encapsulation of drugs is particularly extensive (I. Fichtner, D. Arndt: Stand und Perspektiven der Liposomenforschung (Status and Perspective of Liposome Research). Pharmazie 44, 752-757 (1989)). In comparison with other carrier systems (nano particles, artificial cells, etc.) liposomes offer various advantages, which can be utilized for such purposes as the encapsulation of cytostatic drugs,
Examples of such advantages are:
the ability to select the composition, charge, size and stability, depending on the particular requirement, PA1 prevention of biological degradation of encapsulated substrates, PA1 the practically nonexisting immunological or toxic reactions, PA1 the frequently altered pharmacokinetics of the liposomal encapsulated substance, PA1 the altered distribution among the organs and the tropism to particular organs, PA1 the possibility of employing different targeting methods (lectins and antibodies.
Due to their amphiphilic character liposomes, can enclose water soluble as well as lipid soluble substances, almost all clinically established cytostatic, as well as some that are still in the development stage have been encapsulated and their physicochemical, biochemical and pharmacological properties have been characterized in comparison to the free compound (literature as above). When liposomally encapsulated antineoplastic drugs are used, a decrease in the toxicity is frequently observed, On the other hand, the therapeutic effectiveness of the liposomal and free form of the drug is approximately the same.